HVDC device for converting between alternating voltages and direct current voltages

ABSTRACT

A device for converting alternating voltage to direct voltage and, conversely, direct voltage into alternating voltage. A series connection between the poles of a direct voltage side has at least four units each having a semiconductor element of turn-off type and a first diode connected in anti-parallel therewith. A first midpoint of the series connection is connected to an alternating voltage phase line and forms a phase output. Second midpoints of the series connection are connected to a midpoint of the direct voltage side through such units. An apparatus is adapted to control the semiconductor elements with a pulse width modulation frequency of at least one order of magnitude higher than the fundamental frequency of the alternating voltage of the phase line and the rest of the semiconductor elements with a frequency substantially lower and within or close to the frequency range of one or a couple of times of the fundamental frequency.

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a device for converting alternatingvoltage into direct voltage and conversely, which comprises a seriesconnection of at least four units each consisting of a semiconductorelement of turn-off type and a first diode connected in anti-paralleltherewith, said series connection being arranged between two poles, apositive one and a negative one, of a direct voltage side of the device,an alternating voltage phase line connected to a first mid point, whichis called phase output, of the series connection between two units whiledividing the series connection into two parts, means adapted to providea mid point between the two poles on said direct voltage side and putthese poles on the same voltage but with opposite signs with respect tothe mid point of the direct voltage side, a second mid point of eachsaid part of the series connection being through a second diode with theconducting direction with respect to the phase output opposite to theconducting direction of the first diode in the unit arranged betweenthis second mid point and the phase output connected to the mid point ofthe direct voltage side and an apparatus for controlling thesemiconductor elements of the units to generate a train of pulses withdetermined amplitudes according to a pulse width modulation pattern onthe phase output of the device by alternatingly connecting thealternating voltage phase line to at least the mid point, the plus poleand the minus pole of the direct voltage side.

Such devices may be used in all kinds of situations, in which directvoltage is to be converted into alternating voltage or conversely, inwhich examples of such uses are in stations of HVDC-plants (high voltagedirect current), in which direct voltage normally is converted into athree-phase alternating voltage or conversely or in so calledback-to-back-stations in which alternating voltage is firstly convertedinto direct voltage and this is then converted into alternating voltage,as well as in SVCs (Static Var Compensator), in which the direct voltageside consists of one or more capacitors hanging freely.

Such converter devices already known have a number of drawbacks, whenthese are used for transmitting high powers, and the present inventionaims at a converter device being well suited to transmit high powers,although the invention is not restricted to this field of use, since aconverter device of this type may very well find other field of uses.However, the case of transmitting high powers will for this reasonhereinafter be discussed for eliminating but not in any way restrictingthe invention.

The device defined in the introduction is a so called multi-levelconverter, since it may deliver at least three different phasepotentials on said phase output. Different types of such multi-levelconverters of this voltage stiff so called VSC-type (Voltage SourceConverter) for high power applications have been described in theIEEE-article IEEE Trans. on Ind. Appln. Vol 32, no 3, 1996, pages509-517. Three different types of multi-level converters are describedtherein, namely multi-level converters with clamping diodes, multi-levelconverters based upon flying capacitors and multi-level converters basedupon cascaded converters. Only the two first ones are suitable fortransmitting active power, such as for example in HVDC- andback-to-back-applications. The greatest problem of multi-levelconverters having clamping diodes is that the diode cost will be veryhigh when the number of levels increases, so that for example in thecase of five levels the number of clamping diodes increases so thatthere is a need of more clamping diodes then said semiconductor elementsof turn-off type. The converter devices with flying capacitors requirefor sure no clamping diodes, but they require instead a large number ofcapacitors, and the capacitor size increases with a comparatively largefactor when the number of levels is increased, in which this factor isfor example five to six when it is changed from three to five levels.Accordingly, this solution is also very costly.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a converter device ofthe type defined in the introduction, which is well suited for highvoltage and high power applications and in which the drawbacks mentionedabove of such devices already known are reduced to a large extend,primarily at an increased number of levels of the converter.

This object is according the invention obtained by connecting asemiconductor element of turn-off type in anti-parallel with each ofsaid second diodes in a device of the type mentioned in theintroduction, and that the apparatus is adapted to control thesemiconductor elements of the units between the two second mid points tobe turned on and turned off with a pulse width modulation frequency ofat least one order of magnitude higher than the fundamental frequency ofthe alternating voltage of said alternating voltage phase line and tocontrol the semiconductor elements connected in anti-parallel with saidsecond diodes and in the units between the respective second mid pointand the respective pole to be turned on and turned off with a frequencybeing substantially lower than said pulse width modulation frequency andwithin or close to the frequency range one or a couple of times saidfundamental frequency.

By arranging a semiconductor element of turn-off type in this way inanti-parallel with said second diodes it is possible to also control theconnection of the mid point of the direct voltage side to the second midpoint, and it gets possible to obtain a desired pulse width modulationpattern at the connection of the phase line to the phase output byturning these semiconductor elements on and off as well as thosearranged between said second mid point and the respective pole with acomparatively low frequency in the order of the fundamental frequency ofthe alternating voltage of the alternating voltage phase line, whereasthe semiconductor elements of the “inner” units are turned on and offwith a significantly higher frequency, more exactly the so called realpulse width modulation frequency. The frequency through which thesemiconductor elements first mentioned are switched may for example be50 or 60 Hz, while the pulse width modulation frequency is typically 1-2kHz. This means that totally different, more exactly lower, demands aremade upon the semiconductor elements first mentioned, which have not tobe turned on and turned off with any high frequency, which means thatfor this semiconductor elements having a considerable better ability tohold high voltages may be used, since such high voltage semiconductorelements cannot take high frequencies without unacceptably highswitching losses. Voltages in the order of 10-400 kV are normallyhandled in devices of this type, and this requires then a seriesconnection of a higher number of semiconductor elements within each saidunit for a series connection of a higher number of said units so as todistribute the voltage these have to hold in the blocking state among ahigh number of such semiconductor elements. Thus, in the present case itwill be possible to use a lower number of semiconductor elementsconnected in series between said second mid point and the respectivepole, since these may be of high voltage type, for example hold 4-6 kVinstead of 2-3 kV, which means a considerable saving of costs andsimplifies the control of the device. Semiconductor elements with asmaller component area may alternatively be used, which have a higherthermal resistance, but which are available to a low cost, may be usedfor these semiconductor elements switched comparatively seldom. The samecondition is valid for the semiconductor elements connected inanti-parallel with the second diodes and switched with a low switchingfrequency.

According to a preferred embodiment of the invention the apparatus isadapted to control the semiconductor elements connected in anti-parallelwith said second diodes and in the units between the respective secondmid point and the respective pole with a frequency coinciding with saidfundamental frequency in absence of voltage harmonics in the alternatingvoltage phase line. A large difference in frequency between the controlof these semiconductor elements and the other semiconductor elements isobtained by this and the advantages mentioned above of the inventionwith respect to the devices already known will by this be veryremarkable. The apparatus is advantageously adapted, when said voltageharmonics occur to optionally carry out one or several additionalswitchings of the semiconductor elements connected in anti-parallel withsaid second diodes and those in the units between the respective secondmid point and the respective pole within a fundamental frequency period,in which a switching is defined as comprising a turn-off and turn-on. Itmay in this way be compensated for such instabilities, in which thefrequency in question may during a very short period of time become forexample three times said fundamental frequency, but it is stillconsiderably lower than the pulse width modulation frequency.

According to another preferred embodiment of the invention the apparatusis adapted to control the semiconductor elements of the units and thesemiconductor elements connected in anti-parallel with said seconddiodes to alternatingly connect the alternating voltage phase line to anodd number of different levels, in which one of them is the mid point ofthe direct voltage side and just as many are positive as negative, inwhich said number is n, which is at least five, that at least (n−1)/2 ofsaid units are connected in series between the second and the first midpoint, that it comprises (n−3)/2 so called flying capacitors and thateach said flying capacitor is connected with one pole thereof to a midpoint of said series connection, which is located between the phaseoutput and the second mid point on the opposite side of the phase outputwith respect to the connection mid point belonging to the opposite polethereof and has at least one unit between itself and the second midpoint and another unit between itself and another capacitor connectionor the phase output. Such a multi-level converter device with a highernumber of levels than another converter device, which has for examplethree levels, results in a better adaptation of the pulse widthmodulation pattern to the sinus wave desired to be obtained downstreamof an inductor or transformer arranged in said alternating voltage phaseline, so that the harmonics generated during the conversion are reducedor the size of these inductors and/or filters for extinguishing suchharmonics may be reduced, lower voltage differentials may be obtainedfor said inductors or transformers, so that stresses thereon may bereduced and these may be made less costly, and lower switching lossesmay be obtained. Such converter devices with a higher number of levelsand the advantages associated therewith may according to thisadvantageous embodiment of the invention be obtained in a simple way andto a low cost. The advantages of the lower frequency of thesemiconductor elements arranged between the respective second mid pointand the respective pole as well as the semiconductor elements connectedin anti-parallel with said second diodes have been discussed above. Inaddition thereto, this way to arrange a flying capacitor is veryadvantageous with respect to the arrangement of flying capacitors of thesecond type mentioned above of converter devices based upon flyingcapacitors, since a flying capacitor (s) are connected in such a waythat they across the poles thereof will have a considerably lowervoltage than in the case of the flying capacitors of the devices alreadyknown, more exactly the voltage across the flying capacitor with thehighest voltage thereacross is in the invention preferably not more thanhalf the voltage across the entire series connection, which is of agreat importance, since the power to be handled by a capacitor isproportional to the square of the voltage, so that the present inventionenables a use of flying capacitors while avoiding the large number ofclamping diodes which would be necessary in the case of a converterdevice of the type first mentioned, and the drawbacks of the second typeof converter devices based upon flying capacitors has with respect torequirements of very large capacitors for a large number of levels ofthe converter device are nevertheless avoided. A large advantage of aconverter device according to this embodiment of the invention isaccordingly that it is possible to get a five-level-converter to acomparatively low additional cost with respect to athree-level-converter, and it will be easy to modify athree-level-converter.

According to a preferred embodiment of the invention n is 5 and saidunits are adapted to give the flying capacitor a voltage across thepoles thereof substantially equal to U/4n, in which U is the voltagebetween the two poles of the direct voltage side. The voltage of theflying capacitor may in this way be kept low and the size and the costthereof may be kept at a low level.

According to another preferred embodiment of the invention the apparatusis adapted to control said units, when one pole of said flyingcapacitors is connected to said phase output so that the phase currentpasses said capacitor, to make this connection in one of two ways, whichgives substantially the same phase potential on the phase outletdepending upon the instantaneous real level of the voltage between thepoles of the capacitor, so that the capacitor is upon said connectioncharged for a voltage level thereof lower than desired and dischargedfor a voltage level thereof higher than desired. This process ispossible thanks to the fact that there are two possible states givingalmost the same potential on the phase output, in which one state may beused for charging the capacitor and the other for discharging thecapacitor for a given direction of the phase current. This process meansthat the capacitance value of the capacitor may be kept at a minimum,with a time constant for the charging and the discharging, respectively,which is a suitable factor higher than the period of time during whichthe capacitor is normally switched in each of the positions for a givenswitching frequency.

Further advantages as well as advantageous features of the inventionappear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description ofpreferred embodiments of the invention cited as examples.

In the drawings:

FIG. 1 is a circuit diagram illustrating a voltage stiff forcedcommutated three-level-converter already known connected to analternating voltage network through inductors, in which only one phaseleg is shown,

FIG. 2 illustrates a converter device of three-level-type according to afirst preferred embodiment of the invention, in which this is connectedto a three-phase alternating voltage network through inductors,

FIG. 3 illustrates the construction of the device according to FIG. 2for one phase of the alternating voltage network,

FIG. 4 is a view corresponding to FIG. 3 illustrating afive-level-converter device according to a second preferred embodimentof the invention, and

FIG. 5 is a view corresponding to FIGS. 3 and 4, although somewhatsimplified, of a seven-level-converter device according to a thirdpreferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The converter device shown in FIG. 1 is a so called NPC (Neutral PointClamped)-converter device with clamping diodes of a type well known, andthis figure is shown here only for comparing the design of thisconverter device with a three-level-converter device according to afirst preferred embodiment of the invention, which is shown in FIGS. 2and 3 and now will be described with reference to these two figures.Only a part of the converter device connected to a phase of analternating voltage phase line is shown in FIG. 3, but it is alsopossible that this constitutes the entire converter device, when this isconnected to a one-phase alternating voltage network. The converterdevice is a so called VSC-converter, which has four units 1-4, usuallycalled transistor valves or alternatively thyristor valves, connected inseries between the two poles 5, 6 of a direct voltage side of thedevice. Two capacitors 7, 8 connected in series are arranged betweensaid two poles, and a point 9 (the mid point of the direct voltage side)therebetween is connected to ground through an impedance Z, in whichthis impedance may vary from 0 (=direct grounding of the mid point ofthe direct voltage side) to a value X (=impedance grounding of the midpoint of the direct voltage side, through for example a resistance R oran inductance L) up to a value Xmax (=ungrounded mid point, in which thegrounding is only determined by the stray capacitance between the midpoint of the direct voltage side and ground), so that the potentials+U/2 and −U/2, respectively, are in this way provided at the respectivepole, in which U is the voltage between the two poles 5, 6.

The units 1-4 are each made of a semiconductor element 10-13 of turn-offtype, such as an IGBT or a GTO, and a first diode 14, a so calledfree-wheeling diode, connected in anti-parallel therewith. Although onlyone IGBT or GTO per unit has been shown this may stand for a pluralityof IGBTs or GTOs connected in series and controlled simultaneously,which also normally is the case, since a comparatively high number ofsuch semiconductor elements are required for holding the voltage to beheld by each unit in the blocking state.

A first mid point 15 of the series connection between the two units 2and 3, which constitutes the phase output of the converter, is connectedto an alternating voltage phase line 16 through an inductor 17. Saidseries connection is in this way divided into two equal parts with twounits 1, 2 and 3, 4, respectively, of each such part.

A second mid point 18, 19 of each said part of the series connection isthrough a second diode 20, 21 with a conduction direction with respectto the phase output opposite to the conducting direction of the firstdiode in the unit arranged between this second mid point and the phaseoutput connected to the mid point 9 of the direct voltage side. Asemiconductor element 22, 23 of turn-off type, such as an IGBT, isconnected in anti-parallel with each second diode. It is also here validthat a great number of semiconductor elements may in practice beconnected in series so as to distribute the voltage they have to hold inthe blocking state among each of them, although one single semiconductorelement has been shown in anti-parallel with each diode.

Furthermore, the device has an apparatus 24 adapted to control thedifferent semiconductor elements and by that ensure that the phaseoutput 15 is connected to and receive the same-potential as the pole 5,the pole 6 or the mid point 9 of the direct voltage side. This apparatus24 and the arrangement thereof is here very simplified illustrated, anda separate such apparatus could in practice be arranged on highpotential at each individual unit and these receive control signals froma control apparatus arranged on ground level. The function of theapparatus will be explained further below.

The converter device shown in FIG. 3 differs with respect to theconstruction thereof from the NPC-converter device already known shownin FIG. 1 by the replacement of the two clamping diodes 20′, 21′ by aunit consisting of a semiconductor element of turn-off type and a seconddiode connected in anti-parallel therewith.

By the new characteristic of the invention, i.e. to replace a clampingdiode by a unit comprising a diode and a semiconductor element ofturn-off type connected in anti-parallel therewith, completely newpossibilities to obtain the different voltage levels desired to beobtained on the phase output 15 are obtained. More exactly it ispossible to control the semiconductor elements 11, 12 of the unitsbetween the two second mid points 18, 19 through the apparatus 24 asbefore to be turned on and turned off with a pulse width modulationfrequency, which preferably is in the order of 1-2 kHz and at least anorder of magnitude, usually 20-40 times higher than the fundamentalfrequency of the alternating voltage of substantially sinusoidal designto be obtained on the alternating voltage phase line 16 on the oppositeside of the inductor 17 with respect to the phase output 15. However,through said replacement of the clamping diodes the semiconductorelements 10, 13 of the units located between the respective pole and therespective second mid point have not to be turned on and turned off witha higher frequency than a frequency in the order of said fundamentalfrequency any longer, in which the frequency in question preferably isidentical to said fundamental frequency, but this may also be a multiplethereof, such as for example three times the fundamental frequency,especially when voltage harmonics occur on the alternating voltage phaseline 16, in which the phase voltage may pass zero at more occasions thantwo times per period. The semiconductor elements 22 and 23 are alsocontrolled with the same frequency as the semiconductor elements 10 and13. The advantages of not being forced to control the semiconductorelements 10 and 13 with the same high frequency as the semiconductorelements 11 and 12 appear clearly from the disclosure above. +U/2 isobtained on the phase output 15 by turning the semiconductor elements 10and 11 on, −U/2 is obtained by turning the semiconductor elements 12 and13 on, while the mid point potential may be obtained either throughturning the semiconductor elements 23 and 12 or the semiconductorelements 22 and 11 on.

It is illustrated in FIG. 2 how a converter device according to FIG. 3is designed for converting direct voltage into alternating voltage andconversely between a direct voltage side and an alternating voltagenetwork with three phases 25, 26, 27. A control for each phase is takingplace in accordance with the description made with reference to FIG. 3.

A converter device with respect to one phase according to a secondpreferred embodiment of the invention is illustrated in FIG. 4 in a viewcorresponding to FIG. 3, and this differs from the embodiment accordingto FIG. 3 by the series connection of eight units 28-35 between the twopoles 5 and 6, in which four are arranged on each side of the phaseoutput 15. Furthermore, two units are arranged between the respectivesecond mid point 18, 19 and the respective pole. Moreover, two units36-39 are arranged instead of one such unit between each mid point 18and 19, respectively, and the mid point 9 of the direct voltage side.Finally, a so called flying capacitor 40 is connected with one polethereof to a mid point 41, 42 of the series connection of units, whichis located between said phase output 15 and the second 18, 19 mid pointon the opposite side of the phase output with respect to the connectionmid point belonging to the opposite pole thereof and has a unit betweenitself and the second mid point and a unit between itself and the phaseoutput.

It is possible to obtain five different levels of pulses delivered tothe first mid point 15 in this device, namely +U/2, +U/4, 0, −U/4 and−U/2. It is in this device intended to control the units located betweenthe second mid points 18 and 19 as described above with a pulse widthmodulation frequency and the units 28, 29, 34, 35 located between therespective second mid point and the pole as well as the units 36, 37 and38, 39 are controlled with a considerably lower frequency in the orderof the fundamental frequency of the alternating voltage phase line 16.

The following switch state table indicates the voltage levels obtainableat the first mid point 15 and which switch states preferably should beused so as to obtain these voltage levels.

28 34 and 29 30 31 and 35 33 32 36 and 37 38 and 39 V = U/2 1 1 1 0 0 00 1 V = U/4 1 1 0 0 0 1 0 1 V = U/4 1 0 1 0 1 0 0 1 V = 0 1 0 0 0 1 1 01 V = 0 0 1 1 1 0 0 1 0 V = −U/4 0 1 0 1 0 1 1 0 V = −U/4 0 0 1 1 1 0 10 V = −U/2 0 0 0 1 1 1 1 0 1 and 0 stand in a conventional way forturned on and turned off, respectively.

The following “rules” have been used in this table.

The following couples have the same state:

28, 29 and 38, 39,

34, 35 and 36, 37.

The following couples are complementary:

28, 29 and 36, 37,

34, 35 and 38, 39

28, 29 and 34, 35

30 and 33

31 and 32

36, 37 and 38, 39.

It appear from the table above that the intermediate voltage levels +U/4and −U/4, respectively, which voltages may be obtained by switching inthe flying capacitor 40, which is charged to the voltage U/4, in eitherdirection. This may be obtained in two different ways, which charges ordischarges the flying capacitor 40. Which one of these two ways is theone to be used is determined by the voltage of the flying capacitor 40,so that an apparatus corresponding to the apparatus 24 in FIG. 3controls the different semiconductor elements to choose the wayresulting in a charging of the capacitor when the voltage across thepoles thereof is too low and a discharging thereof when the voltagethereacross is too high with the aim to keep the voltage across thepoles of the capacitor at U/4. The voltage across the capacitor is bythat kept almost constant, which means a low energy content and thecapacitor may by that be made small, i.e. with a low capacitance.

We assume for the sake of exemplifying that the frequency of thealternating voltage on the phase line 16 is for example 50 or 60 Hz andthe converter only has to deliver active or reactive power at thisfundamental frequency. The following is then valid: we assume that theconverter operates with a pulse width modulation frequency(PWM-frequency) of 1-2 kHz. The inner units 30-33 will then have a meanswitching frequency of half the PWM-frequency, i.e. 0,5-1 kHz. The otherunits 28, 29, 34, 35, 36, 37 and 38, 39 will only switch with thefundamental frequency (50 or 60 Hz), and they will do so either at0-current (28, 29 and 34, 35) or at 0-voltage (36, 37 and 38, 39). Thismeans that the switching losses will be kept at a low level. This isvalid especially for the semiconductor elements 28, 29, 34, 35, 36, 37,38, 39. But also the semiconductor elements 30-33 will have a lower meanswitching frequency than the corresponding 3-level-converter, so thathigh voltage semiconductor elements in may be chosen, or alternativelysemiconductor elements with a smaller area and a higher thermalresistance.

A seven-level-converter device constructed in the same way as theconverter device according to FIG. 4 is illustrated in FIG. 5, and thefunction thereof appear from the description of the device according toFIG. 4. Thus, two flying capacitors 43 are arranged for obtainingdifferent voltage levels, in which the voltage across the outercapacitor 40 will be U/3 and across the inner capacitor 43 U/6. It ispossible to continue in this way and by adding further flying capacitorsobtain converter devices with more levels, i.e. 9, 11,.

The valves are advantageously so designed that they give the flyingcapacitors a voltage U_(x) across the two poles thereof of$\frac{x \cdot U}{\left( {n - 1} \right)},$

in which x=1, . . . $\frac{n - 3}{2}$

and U is the voltage across the two poles of the direct voltage side.This means for example in the case of 9 levels U₁=U/8, U₂=2U/8 andU₃=3U/8.

The invention is of course not in any way restricted to the preferredembodiments described above, but many possibilities to modificationsthereof would be apparent to a man with ordinary skill in the artwithout departing from the basic idea of the invention.

The distribution of the units arranged on both sides of the differentmid points of said series connection and between said second mid pointsand the respective pole may for example be different should that bedesired, so that the voltage levels obtained on the first mid point 15have another mutual relation than shown above.

What is claimed is:
 1. A device for converting alternating voltage intodirect voltage, and, conversely, direct voltage into alternating voltagecomprising: a series connection of at least four units, each having aturn-off type semiconductor element and a first diode connectedanti-parallel therewith, said series connection extending between apositive and negative pole of a direct voltage source; an alternatingvoltage phase line connected to a first midpoint dividing the seriesconnection into two parts, said first midpoint constituting a phaseoutput for the series connection; means providing a midpoint between thetwo poles of said direct voltage having a voltage difference with eachpole which is the same magnitude but of opposite sign; a pair of secondmidpoints formed in the series, connected through second diodes to saidmidpoint between said two poles, said second diodes having a conductiondirection with respect to the phase output opposite to the conductingdirection of a first diode in the unit connected between the respectivesecond midpoint and the output phase, each of said second diodes havinga turn-off type semiconductor element connected anti-parallel with eachof said second diodes; and an apparatus for controlling thesemiconductor elements of the units to generate a train of pulsesaccording to a pulse width modulation pattern on the phase output, saidapparatus alternately connecting the alternating voltage phase line tothe midpoint, and the two poles of the direct voltage; said apparatuscontrolling the semiconductor elements of the units between the twosecond midpoints in response to a pulse width modulation frequency of atleast one order of magnitude higher than the fundamental frequency ofthe alternating voltage of the phase line, and controlling thesemiconductor elements connected anti-parallel with said second diodes,and in the remaining of said units, with a frequency substantially lowerthan said pulse width modulation frequency, and substantially the sameas a frequency range one or two times said fundamental frequency.
 2. Adevice according to claim 1, wherein in that said apparatus (24) isadapted to control the semiconductor elements (22, 23) connected inanti-parallel with said second diodes (20, 21) and in the units (1, 4,28, 29, 34, 35) between the respective second mid points and arespective pole with a frequency a multiple of said fundamentalfrequency.
 3. A device according to claim 1, wherein said apparatus (24)is adapted to control the semiconductor elements connected anti-parallelwith said second diodes and in the units between the respective secondmid point and the respective pole with a frequency coinciding with saidfundamental frequency in absence of voltage harmonics in the alternatingvoltage phase line.
 4. A device according to claim 3, wherein saidapparatus (24) provides additional switching of the semiconductorelements connected in anti-parallel with said second diodes and those inthe units between the respective second mid point and the respectivepole within a fundamental frequency period.
 5. A device according toclaim 1, wherein one or more said units connected in series arecontrolled simultaneously through the apparatus (24), each of said unitssemiconductor elements having substantially the same portion of avoltage applied across said series when the semiconductor elements areturned off, two units (1, 2 and 3, 4), respectively are arranged betweenthe phase output (15) and a respective pole (5, 6), a unit (1,4) isarranged between the respective first mid point (18, 19) and therespective pole, that a unit is arranged between the mid point (9) ofthe direct voltage side and said second mid point (18, 19), wherein theapparatus (24) provides an alternating connection of three differentpotential levels to the phase output (15).
 6. A device according toclaim 1, wherein the apparatus (24) controls the semiconductor elements(10-13) of the units and the semiconductor elements (22, 23) connectedin anti-parallel with said second diodes to alternatingly connect thealternating voltage phase line to an odd number n where n is at least 5,of different levels, one of said levels being the mid point of thedirect voltage side, at least (n−1)/2 of said units are connected inseries between the second (18, 19) and the first mid point (15), (n−3)/2flying capacitors (40, 43), are connected to a mid point (41, 42)between units of said series connection connected between the phaseoutput and the second mid points.
 7. A device according to claim 6,wherein n is 5, and that it has one said flying capacitor (40).
 8. Adevice according to claim 7, wherein said units are adapted to providethe flying capacitor (40) a voltage substantially equal to U/4, in whichU is the voltage between the two poles of the direct voltage side.
 9. Adevice according to claim 7, wherein said series connection has eightunits (28-35), in which each unit is adapted to take substantially thesame portion of a voltage applied to one or more such units connected inseries when the semiconductor elements included therein are turned off,that four units (28-35) are arranged between the phase output and therespective direct voltage pole, two units (28, 29, 34, 35) are arrangedbetween the respective second mid point and the respective directvoltage pole, that a unit (31, 32) is arranged between the respectiveconnection (41, 42) of the flying capacitor (40) to the seriesconnection and the phase output (15), and that two units (36-39) areconnected in series between the mid point of the direct voltage side andsaid second mid point.
 10. A device according to claim 6, wherein n is7.
 11. A device according to claim 9, having two flying capacitors (40,43), the inner capacitor (43) connected to said series connectionclosest to the phase output (15) has a voltage of U/6 across the polesthereof and a second, outer capacitor (40) has a voltage U/3 across thepoles thereof, in which U is the voltage between the two poles of thedirect voltage side.
 12. A device according to claim 6, wherein theunits provide the flying capacitors (40, 43) a voltage U_(x) across thetwo poles thereof of${{{\frac{x \times U}{\left( {n - 1} \right)}\text{in which}\quad x} = 1},{\ldots \quad \frac{n - 3}{2}}}\quad$

and U is the voltage across the two poles of the direct voltage side.13. A device according to claim 6, wherein the apparatus (24) controlssaid unit, so that when one pole of said flying capacitors (40, 43) isconnected to said phase output in one of two ways the phase currentpasses said capacitor to provide substantially the same phase potentialon the phase outlet depending upon the instantaneous real level of thevoltage between the poles of the capacitor, so that the capacitor ischarged for a voltage level thereof lower than desired and dischargedfor a voltage level thereof higher than desired.
 14. A device accordingto claim 1, wherein said semiconductor elements are IGBTs (InsulatedGate Bipolar Transistor).
 15. A device according to claim 1, whereinsaid semiconductor elements are GTOs (Gate Turn-Off thyristor).
 16. Adevice according to claim 1, wherein said direct voltage side is formedby a direct voltage network for transmitting high voltage direct current(HVDC) and the alternating voltage phase line belongs to an alternatingvoltage network.
 17. A device according to claim 1, incorporated in aSVC (Static Var Compensatory with the direct voltage side formed bycapacitors hanging freely and the alternating voltage phase lineconnected to an alternating voltage network.
 18. A device according toclaim 1, wherein it has at least two alternating voltage phase lines(16, 25-27) included in a multiple-phase alternating voltage network,each having one of said series connection and second diodes associatedtherewith connected in parallel with each other between said poles ofthe direct voltage side and having semiconductor elements of turn-offtype connected anti-parallel therewith for each phase line.
 19. A deviceaccording to claim 18, wherein the number of phases of the alternatingvoltage network is three.
 20. A device according to claim 2, whereinsaid apparatus is adapted to control the semiconductor elementsconnected anti-parallel with said second diodes and in the units betweenthe respective second mid point and the respective pole with a frequencycoinciding with said fundamental frequency in absence of voltageharmonics in the alternating voltage phase line.